1. Field of the Invention
This application relates generally to a semiconductor device.
2. Description of the Related Art
Semiconductor devices, particularly integrated circuits using MOS transistors, which are field effect transistors having gate electrodes with a MOS (metal-oxide semiconductor) structure, are increasingly being highly integrated. MOS transistors in integrated circuits have been downsized to nano sizes as the integration level is increased. When MOS transistors constitute an inverter circuit (a NOT circuit)—one of the basic types of digital circuits—as those MOS transistors are downsized, problems arise such as difficulty in leaking current control and reduced reliability occurs due to hot carrier effect. In addition, the problem occurs that reducing the area occupied by circuits is impossible due to the necessity of assuring the necessary current quantity. In order to resolve these problems, a surrounding gate transistor (SGT) structure has been proposed having an island-shaped semiconductor layer in which the source, gate and drain are provided on a substrate in the vertical direction and the gate surrounds this island-shaped semiconductor, and CMOS inverter circuits using SGTs have been proposed (for example, see 1. Watanabe, K. Tsuchida, D. Takashima, Y. Oowaki, A. Nitayama, K. Hieda, H. Takato, K. Sunouchi, F. Horiguchi, K. Ohuchi, F. Masuoka, H. Hara, “A Novel Circuit Technology with Surrounding Gate Transistors (SGT's) for Ultra High Density DRAM's”, IEEE JSSC, Vo. 30, No. 9, 1995).
An inverter circuit, which is one of the basic types of digital circuits, consists of a p-channel type MOS transistor (pMOS transistor) and an n-channel type MOS transistor (pMOS transistor). The mobility of holes is half the mobility of electrons. Therefore, in an inverter circuit the pMOS transistor must have a gate width double the gate width of the nMOS transistor. For this reason, a conventional CMOS inverter circuit using SGTs comprises two directly connected pMOS SGTs and one nMOS SGT. In other words, a conventional CMOS inverter circuit using SGTs comprises three island-shaped semiconductors
When composing SRAM (Static Random Access Memory) using CMOS inverter circuits using such SGTs, the SRAM comprises two inverter circuits and two selection transistors. When SRAM is made with a conventional CMOS inverter circuit using SGTs, such must constitute four pMOS SGTs and four pMOS SGTs. In other words, SRAM using conventional CMOS inverter circuits with SGTs comprises a total of eight island-shaped semiconductors. When SRAM using CMOS inverter circuits using SGTs comprises eight island-shaped semiconductor layers, this becomes an impediment to increasing integration of the semiconductor device.